Chromates are chemicals of vital importance. As the raw materials for manufacturing a series of chromium-containing chemicals such as dichromate, chromic oxide and chromium anhydride, chromates are widely used in leather tanning, pigment manufacturing, dyestuff manufacturing, metallurgical industry, materials industry, metal plating, porcelain enamel production, wood preservation and pharmacy.
Manufacturing chromates from chromite ore is an important basic materials industry crossing the chemical and metallurgical industries. The current industrial production method is mainly the roasting of alkali metal carbonates. It is a high-temperature oxidative roasting reaction of chromite ore, alkali metal carbonates, and air in a rotary kiln. The reaction temperature is in the range from 900 to 1200° C. It needs the adding of alkali additives such as limestone, dolomite, and ferric oxide in the reaction process. Primary chromate products are obtained by leaching the roasting products with water. Pure chromate products or other chromium-containing products can be obtained after impurity removal and evaporation crystallization. However, the roasting process produces a great amount of highly toxic chromium-containing waste residues, creates serious environmental pollution, and the method remains a global unresolved environmental problem so far.
Presently, the traditional soda-ash roasting technology is still in use in many countries including China. Calcium-containing auxiliaries, mostly alkali compounds, are added in the process of roasting. The function of the calcium-containing auxiliaries is to reduce the mass transfer resistance in the liquid film of chromite ore during the roasting process, consequently strengthening the mineral decomposition process, reducing the consumption of alkali as raw materials, improving the product quality, and eliminating the unfavorable effects on roasting, separation and product quality of the acidic components such as aluminum oxide and silicon oxide in the chromite ore. In this process, the amount of calcium-containing auxiliaries is twice as much as the amount of chromite ore. The reaction temperature is as high as 1200° C. However, the recovery rate of chromium is below 80 percent and 2.5 to 3.0 tonnes of highly toxic chromium-containing residues are produced with the production of 1.0 tonne of chromate products. The currently-used disposal method of the chromium-containing residues is mainly landfilling, which not only wastes the chromium resources and calcium-containing auxiliaries but also results in great environmental threats potentially.
To minimize the discharging amount of chromium-containing residue and maximize the recovery rate of chromium, investigations on less-calcium roasting and calcium-free roasting processes were carried out in many countries. The patent technology of calcium-free roasting, which has been industrialized in developed countries including UK and USA, and less-calcium roasting technology are both improved roasting technology. In this technology the recovery rate of chromium is increased by substituting a certain amount of chromium-containing residues for calcium-containing auxiliaries and prolonging the staying time of the chromium-containing residue in the rotary kiln so as to further oxidize the chromium in the chromium-containing residue. In the meanwhile, the amount of auxiliaries can be reduced, even to zero, and therefore decreases the discharging amount of chromium-containing residues.
As the most advanced roasting technology presently, the calcium-free roasting technology achieves higher chromium recovery rate and less chromium-containing residue discharging amount than the traditional soda-ash roasting technology. But the total chromium recovery rate of calcium-free technology is only 90%, and 0.8 tonnes of chromium-containing waste residues are discharged with the production of 1.0 tonne of products, and the environmental pollution problem of chromium-containing residues remains unresolved.
Manufacturing chromates via molten salt method has been reported since 1980s. In this method, chromite ore is oxidized and decomposed by air in the molten salt of alkali metal hydroxide or carbonate. The amount of alkali metal hydroxide or carbonate is higher than their stoichiometric amount for decomposing chromite ore. The reaction temperature is 500 to 900° C., higher than the melting point. This method does not involve the key technology of continuous oxidation and phase separation in molten salt media and therefore is not industrially applicable.
In a summary, all the methods available for producing chromates produces a great amount of chromium-containing waste residues and results in serious environmental pollution. Furthermore, none of these methods achieves an approximately 100% of chromium recovery rate and the comprehensive utilization of other valuable components in the chromite ore such as aluminum, magnesium and iron, therefore resulting in resource waste of chromite ore.